Until recently, the telecommunication services included in an automobile were limited to a few systems, mainly analog radio reception (AM/FM bands). The most common solution for these systems is the typical whip antenna mounted on the car roof. The current tendency in the automotive sector is to reduce the aesthetic and aerodynamic impact of such whip antennas by embedding the antenna system in the vehicle structure. Also, a major integration of the several telecommunication services into a single antenna is especially attractive to reduce the manufacturing cost or the damage due to vandalism and car wash systems.
Antenna integration is becoming more and more necessary due to a deep cultural change towards an information society. The Internet has evoked an information age in which people around the globe expect, demand, and receive information. Car drivers expect to be able to drive safely while handling e-mail and telephone calls and obtaining directions, schedules, and other information accessible on the world wide web (WWW). Telematic devices can be used to automatically notify authorities of an accident and guide rescuers to the car, track stolen vehicles, provide navigation assistance to drivers, call emergency roadside assistance, and provide remote engine diagnostics.
The inclusion of advanced telecom equipment and services in cars and other motor vehicles is very recent, and was first limited to top-level, luxury cars. However, the fast reduction in both equipment and service costs are bringing telematic products into mid-priced automobiles. The massive introduction of a wide range of such new systems would generate a proliferation of antennas upon the bodywork of the car, in contradiction with the aesthetic and aerodynamic trends, unless an integrated solution for the antennas is used.
Patent PCT/EPOO/00411 proposed a new family of small antennas based on a set of curves, referred to as space-filling curves. An antenna is said to be a small antenna (a miniature antenna) when it can fit into a small space compared to the operating wavelength. It is known that a small antenna features a large input reactance (either capacitive or inductive) that usually has to be compensated for with an external matching/loading circuit or structure. Other characteristics of a small antenna are its small radiating resistance, small bandwidth and low efficiency. Thus, it is highly challenging to pack a resonant antenna into a space that is small in terms of the wavelength at resonance. The space-filling curves introduced for the design and construction of small antennas improve the performance of other classical antennas described in the prior art (such as linear monopoles, dipoles and circular or rectangular loops).
The integration of antennas inside mirrors has been proposed. U.S. Pat. No. 4,123,756 is one of the first to propose the utilization of conducting sheets as antennas inside of mirrors. U.S. Pat. No. 5,504,478 proposed the use of the metallic sides of a mirror as an antenna for a wireless car aperture. Others configurations have been proposed to enclose a wireless car aperture, garage door opener or car alarm (U.S. Pat. No. 5,798,688) inside the mirrors of motor vehicles. Obviously, these solutions propose a specific solution for determinate systems, which generally require a very narrow bandwidth antenna, and do not offer a full integration of basic service antennas.
Other solutions were proposed to integrate the AM/FM antenna into the thermal grid of the rear windshield (Patent WO95/11530). However, this configuration requires an expensive electronic adaptation network, including RF amplifiers and filters to discriminate the radio signals from the DC source, and is not adequate for transmissions such as telephony signals because of its low antenna efficiency.
One of the substantial innovations introduced by the present invention is the use of a rearview mirror to integrate all basic services required in a car, such as radio-broadcast, GPS and wireless access to cellular networks. The main advantages of the present invention with respect to the prior art include a full antenna integration with no aesthetic or aerodynamic impact, second a full protection from accidental damage or vandalism, and a significant cost reduction.
The utilization of microstrip antennas is known in mobile telephony handsets (See, Paper by K. Virga and Y. Rahmat-Samii, “Low-Profile Enhanced-Bandwidth PIFA Antennas for Wireless Communications Packaging”, published in IEEE Transactions on Microwave theory and Techniques in October 1997), especially in the configuration denoted as PIFA (Planar Inverted F Antennas). The reason for the utilization of microstrip PIFA antennas resides in their low profile, low fabrication costs, and easy integration within the hand-set structure. However, this antenna configuration has not been proposed for use in a motor vehicle. Several antenna configurations claimed by the present invention for the integration of a multiservice antenna system inside of an interior rearview mirror include the utilization of PIFA antennas.
One of the miniaturization techniques used in the present invention is based, as noted above, on space-filling curves. In a particular case of the antenna configuration proposed in this invention, the antenna shape could also be described as a multi-level structure. Multi-level techniques have already been proposed to reduce the physical dimensions of microstrip antennas (PCT/ES/00296).